1. Field of the Invention
The invention relates to a lock stitch sewing machine, and more particularly relates to an automatic thread tension adjusting device of the lock stitch sewing machine.
2. Prior Art
It is generally known that a lock stitch sewing machine has a plurality of thread tension adjusting mechanisms, each of which including a pair of disks which are adjustably pressed against each other by means of a compression spring to give the thread an optional tension in dependence upon the work to be stitched, the thread tension being automatically adjusted by means of actuators including motors, the rotation of which is controlled by a control device. On the other hand, the lock stitch sewing machine has a device which is manually operated to release the thread tension disks. However, it requires a considerably great force to operate the thread tension releasing device to release the thread tension disks at a time against the total spring action of so many compression springs especially when the thread tension is set high with the compression springs strongly compressed. This detracts from the operation ability of the sewing machine when the user operates the thread tension releasing device to release the thread tension.